18sf does not need trim tabs. You want a lighter weight motor . . . even with a 25" transom. Fuel burn with a 2.0L 150 Merc is 3 - 3.5mpg depending on your speed/load. There are lots of reports of 20sf's with a 140 zuke of 4.5 - 5mpg cruise with a top speed of approx 40mph.
2.0L 150 with 17P prop tops out close at about 38/40mph
2.4L 175 with a 19P with top out 45/46'ish.
Performance is very close to the 20 . . . actually I got faster speeds out of my 20 seafari with a bracket over the 18sf.
